The invention relates to aluminoborosilicate glasses which contain alkaline earth metals. The invention also relates to uses of these glasses.
When energy is being obtained by means of photovoltaics, the property of certain semiconducting materials of absorbing light from the visible spectral region as well as the near UV or IR to form free charge carriers (electron/hole pairs) is utilized. If there is an internal electric field in the solar cell, produced by a pn junction in the photoactive semiconductor material, these pairs can be spatially separated using the diode principle, leading to a potential difference and, given suitable contacts, to the flow of current. Solar cell systems which are currently commercially available contain, as photoactive material, almost exclusively crystalline silicon. This is produced as what is known as xe2x80x9csolar grade Sixe2x80x9d, inter alia, as a waste material during the production of high-purity silicon single crystals for complex integrated components (chips).
The possible applications for photovoltaic installations can be roughly divided into two groups. These are, firstly, applications which are not connected to the mains, which are used in remote areas on account of the lack of energy sources which are relatively easy to install. By contrast, solutions which are connected to the mains and in which solar energy is fed into an existing fixed mains remain uneconomical, on account of the high cost of solar current, and are therefore relatively rare.
Therefore, the future market development of photovoltaics, in particular for solutions which are connected to the mains, is highly dependent on the potential for reducing costs in the production of solar cells. The implementation of thin-film concepts is considered to offer great potential. In this case, photoactive semiconductor materials, in particular highly absorbent compound semiconductors, are deposited in layers which are a few xcexcm thick on substrates which are as inexpensive as possible and are able to withstand high temperatures, for example glass. The possibilities for reducing costs lie primarily in the low consumption of semiconductor material and the excellent possibilities for automation during the production compared to wafer Si solar cell production.
Solar cells based on the II-VI compound semiconductor CdTe are a promising thin-film concept. This material satisfies essential conditions, such as a band gap which is well matched to the solar spectrum, high absorption of the incident light and very good chemical stability of the compound.
The same is true of the compound semiconductor Cu(In,Ga) (S,Se)2, (xe2x80x9cCISxe2x80x9d). Compared to the first example, this is also more environmentally friendly, since it does not contain any Cd.
Thin polycrystalline films of CdTe can be produced by a range of methods (vapor deposition, screen printing, sublimation, spray pyrolysis, electrodeposition), but only in p-conducting form. To obtain a pn junction, what is known as a heterojunction has to be produced using a different n-conducting material, e.g. CdS.
In addition to the substrate technologies which are in widespread use in thin-film photovoltaics (semiconductor resting on bases made from materials such as glass, metal, plastic, ceramic), having said layers and a covering glass, with the light acting through the covering glass, a superstrate arrangement has also established itself in particular in CdTe photovoltaics. In this arrangement, the light from impingement on the semiconductor layer initially passes through the support material. This eliminates the need for the covering glass, which has advantages in terms of costs. To achieve high efficiencies, it is necessary for substrates of this type to have a high transparency in the VIS/UV region of the electromagnetic spectrum, which makes the use of glass a suitable solution. For example, even semitransparent glass ceramics are unsuitable, partially for cost reasons caused by the ceramicizing process.
Further demands on the substrate/superstrate material result from the structure of the solar cell and the temperature conditions during the process used for deposition of the CdTe film. With a view to achieving rapid deposition rates for good-quality CdTe, high temperatures, generally of over 650xc2x0 C., are required. Accordingly, the substrate glasses should have a sufficiently high ability to withstand thermal loads, i.e. the transformation point Tg of the glasses should be over 660xc2x0 C. To prevent flaking of the semiconductor layer during the cooling which follows the coating process, the glasses must also be matched to the thermal expansion of CdTe (xcex120/300≈5-6*10xe2x88x926/K). In the case of the CIS technology, in addition to the high Tg ( greater than 650xc2x0 C.), a coefficient of thermal expansion xcex120/300 which is matched to the Mo layer functioning as electrode, of 4.5-5.0*10xe2x88x926/K, is required. The soda-lime glass which has previously been used in no way satisfies these requirements, having an xcex120/300≈9*10xe2x88x926/K and a Tg of approx. 520xc2x0 C.
Furthermore, the glasses are to be sufficiently mechanically stable and chemically resistant to water and also to any reagents used in the production process, in particular in the; case of the superstrate concept, in which there is no covering glass protecting the solar module from environmental influences. For example, soda-lime glasses only have a hydrolytic resistance belonging to Hydrolytic Class 3. Furthermore, it should be possible to economically produce the glasses in sufficient quality in terms of having no or few bubbles and crystalline inclusions.
Similar demands are also imposed on glasses for lamp bulbs:
The glasses have to be able to withstand high thermal loads, since high bulb temperatures generally occur in operation. The glasses must be sufficiently resistant to devitrification to be suitable for tube drawing. For use as lamp bulb glass for lamp bulbs which include molybdenum components as electrode or supply conductor material, the thermal expansion of the glasses has to be matched to that of molybdenum (xcex120/300=5.0*10xe2x88x926/K), so that a sealed, stress-free fusion between the metal and the glass is achieved. For this application too, the glasses must be as free from bubbles as possible. Moreover, glasses for halogen lamps must be substantially free of alkali metals, since alkali metal ions disrupt the regenerative halogen cycle of the lamp.
This profile of requirements is best satisfied by aluminoborosilicate glasses which contain alkaline earth metals but little if any alkali metal. However, the known glasses for display or solar cell substrates which are described in the following documents still have drawbacks in terms of their chemical and physical properties and/or their formation options and fail to satisfy the full range of demands.
Numerous documents describe glasses with relatively high B2O3 contents, for example DE 196 01 922 A, JP 58-120 535 A, JP 60-141 642 A, JP 8-295 530 A, JP 9-169 538 A, JP 10-59 741 A, JP 10-722 37 A, EP 714 862 A1, EP 341 313 B1, U.S. Pat. No. 5,374,595, DE 197 39 912 C1. These glasses do not have the required high transformation temperatures and/or have coefficients of expansion which are too low for the applications which are preferred in this document.
By contrast, B2O3-free glasses are described in U.S. Pat. No. 4,607,016, JP 61-236 631 A and JP 61-261 232 A. The absence of B2O3 means that the glasses are difficult to melt and tend towards devitrification. The glasses mentioned in WO 97/30001 also do not contain any B2O3.
DE 44 30 710 C1 describes borosilicate glasses with a low boric acid content and high SiO2 contents ( greater than 75% by weight) which means that they are highly viscous even at high temperatures and can only be melted and refined at considerable cost. Moreover, these glasses, with transformation temperatures Tg of between 500 and 600xc2x0 C., have only a relatively low thermal stability.
DE 196 17 344 C1 and DE 196 03 698 C1, in the name of the applicant, have disclosed alkali-free, tin-containing glasses with a coefficient of thermal expansion xcex120/300 of approx. 3.7*10xe2x88x926/K and very good chemical stabilities. They are suitable for use in display technology. However, since they inevitably contain at least 1 to 2% by weight of the network modifier ZnO, they are not optimally suitable in particular for processing on a float installation.
The Pb-containing glasses which also have a relatively high Zn content (xe2x89xa73.5% by weight) described in JP 61-295 256 A are also rather unsuitable for the float process, since, if the concentration is too high, it is easy for deposits of ZnO and PbO or Pb to form on the glass surface in the reducing forming-gas atmosphere as a result of evaporation followed by condensation.
Transparent glass ceramics, which are suitable, inter alia, for flat displays and solar cells, are described in JP 3-164445 A. The examples listed have high Tg values of  greater than 780xc2x0 C. and are well matched to CdTe in terms of their thermal expansion. However, on account of their very high zinc contents they are unsuitable for the float production process. The same applies to the transparent mullite-containing glass ceramics, which are doped with at most 1% by weight of chromium, from EP 168 189 A2 and the transparent garnet glass ceramics from JP 1-208343 A with possible applications in solar collectors. However, the high transparency required for use as superstrate in CdTe solar cell systems is not ensured either by glass ceramics, which, depending on the grain size of the crystallites, have a transmission which is lower than that of glasses, or by milky-white opal glasses as are described in FR 2126960.
JP 9-48632 A describes alkali-free aluminoborosilicate glasses which contain alkaline earth metals, with maximum alkaline earth metal oxide contents of 20% by weight. These glasses also have coefficients of thermal expansion which are too low for the applications preferred in this document. DE 196 80 966 T1 and DE 196 80 967 T1 describe alkali-free glass substrates which contain little or no MgO. Their glasses with preferred alkaline earth metal contents of at most 20% by weight also have an insufficiently high expansion.
By contrast, glasses which contain little or no alkali metal but have an excessively high expansion are known from WO 96/9259, JP 9-255354 A, JP 9-255355 A, JP 9-255 356 A and U.S. Pat. No. 5,741,746. The glasses described in WO 96/9259, with at least 45.5% by weight of RO (at least 25.5% by weight of BaO) have a high alkaline earth metal oxide content, as do those described in JP 9-255 354 A (BaOxe2x89xa78% by weight) and U.S. Pat. No. 5,741,746 (BaOxe2x89xa714% by weight), while the glasses described in JP 9-255 355 are low in SiO2.
A substrate glass for applications in LCD and solar cell technology is described in U.S. Pat. No. 4,994,415. The glass, which does not contain any alkali metals or magnesium, includes high levels of BaO, at more than 10% by weight, and therefore ought to be highly resistant to devitrification. However, the density of the glass is high on account of this component. Similar statements are true of the BaO-rich glasses described in U.S. Pat. No. 5,326,730 (12-19 mol % of BaO).
If the BaO contents are too low, there is an increased tendency toward crystallization. This is true not only of the glasses described in WO 98/27019, which contain  less than 3% by weight of SrO+BaO, but also, in particular, of the glasses described in EP 510 544 B1, which contain no barium and have a difference between the working temperature (temperature at the viscosity 104 dPas) and the upper devitrification limit (liquidus temperature) which is unfavorable for the production process. JP 10-45422 A and JP 9-263 421 A describe glasses with alkaline earth metal contents of at most 20 or 22 mol %, respectively, and BaO contents of at most 1 mol %, preferably without any BaO, these glasses also having relatively low SrO contents. The glasses have very high temperatures at the viscosities 104 dpas and 102 dPas, which places very high demands on the tank furnace and manifold material, so that the glasses cannot be produced at low cost. The glasses described in JP 4-175242 A also have relatively low SrO contents (1-9 mol %), but their BaO contents, which are likewise up to 9 mol %, are also rather high.
JP 10-25132 A describes glasses which are refined by means of a combined sulfate/chloride refining. The compositions vary over a wide range but include only at most 10% by weight of the component SrO, which is only optional as is the case for all alkaline earth metal oxides. Their B2O3 content is very high, at up to 20% by weight.
DE-A 1596 767 has already described aluminosilicate glasses for tungsten-iodine lamps which are substantially free of alkali metal oxides. The glasses have an alkaline earth metal oxide content of between 10 and 25% by weight, which can be combined as desired from MgO, CaO, SrO, BaO. According to the examples, it is composed of CaO and BaO and, if appropriate, MgO.
The glasses described in JP 1-126239 A may also vary considerably in terms of the content of the constituent CaO and the optional constituents SrO and BaO, the sum of these three components ranging from 12-25 mol %. The molar ratio between the sum of the said alkaline earth metal oxides, on the one hand, and this sum+Al2O3, on the other hand, is greater than 0.4 and less than 0.6.
On account of their high ratio of network-forming agents to alkaline earth metal oxides in combination with low B2O3 contents, the alkali-free glasses described in EP 0 528 149 B1 should be sufficiently thermally stable for high-temperature coating processes. With alkaline earth metal oxide contents of between 23 and 28 mol %, they encompass a range which can be used to achieve widely varying expansion coefficients. The dominant alkaline earth metal oxide is CaO, while MgO is not present or is present in only small amounts.
U.S. Pat. No. 5,116,789 and EP 0 527 320 B1 have disclosed MgO-free glasses, the dominant alkaline earth metal oxide in which is SrO (15-26 or 21-26 mol %, respectively) JP 9-12333 A has disclosed glasses for hard disk substrate which are rather low in SrO and high in CaO. The alkali-free aluminosilicate glasses for flat displays which are described in EP 672 629 A2 and U.S. Pat. No. 5,508,237 have similar maximum SrO contents. These documents show various composition ranges with different coefficients of thermal expansion. Allegedly, it is supposed to be possible to produce the glasses not only by the overflow-fusion drawing process but also by the float process, yet this is not possible using the refining agents As2O3 and Sb2O3 which are mentioned by way of example and the optional glass components Ta2O5 and Nb2O5, on account of the ease with which they can be reduced.
The sodium-containing glasses described in JP 4-83733 A comprise at least 80% by weight of SiO2, Al2O3, Na2O and MgO. With this basic composition, it is not possible to achieve a thermal expansion of approx. 5-6*10xe2x88x926/K with, at the same time, a high transformation temperature, or else it is possible to achieve this only by accepting drawbacks in terms of other properties.
It is an object of the invention to provide glasses which satisfy the abovementioned physical and chemical demands imposed on glass substrates for thin-film photovoltaic technologies based on compound semiconductors, in, particular based on the II-VI semiconductor CdTe or CIS, glasses which have a thermal stability which is sufficient for high-temperature deposition processes, i.e. a transformation temperature Tg of at least 660xc2x0 C., which have a working temperature range which is appropriate to the process and have a high devitrification stability and also a high quality with regard to the low level of bubbles and a chemical stability which at least matches that of soda-lime glasses.
This object is achieved by the aluminoborosilicate glasses which contain alkaline earth metals in accordance with claim 1.
The glasses contain balanced levels of the network-forming agents SiO2 and Al2O3, with relatively small amounts of the network-forming agent B2O3. In this way, a high thermal stability of the glass combined with melting and working temperatures which remain low is achieved. To achieve the desired coefficient of thermal expansion (xcex120/300 between 4.5 and 6.0*10xe2x88x926/K), the maximum content of SiO2+Al2O3+B2O3 is at most  less than 80% by weight, preferably at most 74% by weight.